linux-kernel - Re: [PATCH V3] bcachefs: Add journal v2 entry nr value check

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Message-ID: <20240821033059.2244258-1-lizhi.xu@windriver.com>
Date: Wed, 21 Aug 2024 11:30:59 +0800
From: Lizhi Xu <lizhi.xu@...driver.com>
To: <kent.overstreet@...ux.dev>
CC: <linux-bcachefs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <lizhi.xu@...driver.com>,
        <syzbot+47ecc948aadfb2ab3efc@...kaller.appspotmail.com>,
        <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH V3] bcachefs: Add journal v2 entry nr value check

On Tue, 20 Aug 2024 23:16:50 -0400, Lizhi Xu wrote:
> > On Tue, 20 Aug 2024 23:00:05 -0400, Kent Overstreet wrote:
> > > > When the nr value of a signle entry or their sum overflows, it will
> > > > cause the value of ja->nr to be incorrect, this will result in the
> > > > allocated memory to ja->buckets being too small, leading to out of
> > > > bounds access in bch2_dev_journal_init.
> > > >
> > > > Reported-by: syzbot+47ecc948aadfb2ab3efc@...kaller.appspotmail.com
> > > > Signed-off-by: Lizhi Xu <lizhi.xu@...driver.com>
> > > > ---
> > > >  fs/bcachefs/journal_sb.c | 16 +++++++++++++++-
> > > >  1 file changed, 15 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/fs/bcachefs/journal_sb.c b/fs/bcachefs/journal_sb.c
> > > > index db80e506e3ab..230ed99130e4 100644
> > > > --- a/fs/bcachefs/journal_sb.c
> > > > +++ b/fs/bcachefs/journal_sb.c
> > > > @@ -107,6 +107,7 @@ static int bch2_sb_journal_v2_validate(struct bch_sb *sb, struct bch_sb_field *f
> > > >  	unsigned nr;
> > > >  	unsigned i;
> > > >  	struct u64_range *b;
> > > > +	u64 total_nr = 0, entry_nr;
> > > >
> > > >  	nr = bch2_sb_field_journal_v2_nr_entries(journal);
> > > >  	if (!nr)
> > > > @@ -117,8 +118,21 @@ static int bch2_sb_journal_v2_validate(struct bch_sb *sb, struct bch_sb_field *f
> > > >  		return -BCH_ERR_ENOMEM_sb_journal_v2_validate;
> > > >
> > > >  	for (i = 0; i < nr; i++) {
> > > > +		entry_nr = le64_to_cpu(journal->d[i].nr);
> > > > +		if (entry_nr > UINT_MAX) {
> > > > +			prt_printf(err, "Journal v2 entry d[%u] nr %llu overflow\n",
> > > > +				i, entry_nr);
> > > > +			goto err;
> > > > +		}
> > >
> > > This check is unnecessary; we know the sum can't overflow a u64 because
> > > we're also checking that the entries are nonoverlapping.
> > You didn't read my previous email carefully.
> > In this issue, journal->d[0] is 7, journal->d[1] is 18446744073709551615,
> > so the sum of their u64 type values will definitely overflow.
> 
> It doesn't matter. We're already checking that the entries are
> nonoverlapping, and within the range of [1, nbuckets), so total_nr can't
> overflow nbuckets, much less an s64 (not that that matters).
Are you sure? Or did I not express myself clearly? See the actual running results below:
le64_to_cpu(journal->d[1].nr) + le64_to_cpu(journal->d[0].nr) = 7 + 18446744073709551615 will overflow.

When a u64 overflow occurs, total_nr will not be greater than UINT_MAX,
so it is not enough to only calculate the total nr of entry to determine.

Note: u64 contains 0 ~ 18446744073709551616.

BR,
Lizhi